The present invention relates to the electroless deposition of copper onto a substrate having a conductive surface and, more particularly, to the electroless deposition of copper, in the presence of nickel or cobalt ions and using a hypophosphite reducing agent, onto a substrate having a conductive surface.
The electroless deposition of metals such as nickel or copper or alloys of these metals with other elements onto suitably prepared substrate surfaces, both conductive and non-conductive, has been known and practiced for some time now. The most commonly practiced method of electroless deposition involves chemical reduction, i.e., wherein deposition takes place by the action of a reducing agent on dissolved metal in the presence of a substrate composed of an inherently catalytic material or a substrate catalyzed by some form of pretreatment.
In the electroless deposition of nickel or nickel alloys via electroless deposition by chemical reduction, the electroless depositing solution generally contains a source of nickel ions, a reducing agent such as a hypophosphite compound, a complexing agent to prevent precipitation of metal ions from solution and an acid or alkaline pH adjusting compound. Electroless nickel solutions of this type are "autocatalytic", i.e., they result in deposition of a coating which itself is catalytic to continued deposition. This permits continuous depositing of nickel or nickel alloy to produce coatings of considerable thickness so long as the concentrations of required ingredients in the electroless solution are maintained.
It was recognized that, for many applications, the use of copper as a conductive coating on a substrate would be preferable or superior to nickel or nickel alloy coatings. Electroless copper depositing solutions relying upon chemical reduction processes were developed which were autocatalytic and which relied upon formaldehyde as the reducing agent. The use of formaldehyde, however, is attendant with a number of problems, not the least of which relate to the potential toxicity of its vapors. Nevertheless, the use of highly alkaline formaldehye electroless copper solutions was essentially the standard in commercially practiced electroless depositing up until the late 1970's, and still is in substantial use at the present time.
A significant advance in the art was achieved in U.S. Pat. No. 4,209,331 wherein a formaldehyde-free electroless copper depositing solution and process employing a hypophosphite reducing agent was disclosed in which the pH of the solution is tailored to the particular complexing agent employed.
The electroless copper solutions disclosed in U.S. Pat. No. 4,209,331, in contrast to the prior formaldehyde-based solutions, are non-autocatalytic, i.e., once the substrate is coated with a thin layer of the deposited copper metal, the deposition reaction stops or becomes uneconomically slow. To a very large degree this can be highly advantageous since another consequence of employing non-autocatalytic solutions is that the depositing can be restricted only to those areas of the substrate or work-piece which have been suitably pretreated (i.e., catalyzed). In contrast, the autocatalytic formaldehyde baths often deposit copper metal on every surface, e.g., tank walls, filters and associated equipment. Nevertheless, it was perceived that in some situations it would be desirable to deposit thicker coatings of copper on the substrate than could be achieved using the process and non-autocatalytic solutions of U.S. Pat. No. 4,209,331.
One method for achieving this result is set forth in U.S. Pat. No. 4,459,184. There, a non-autocatalytic electroless copper solution using hypophosphite as a reducing agent, i.e., as set forth in U.S. Pat. No. 4,209,331, is used to deposit a thin layer of copper metal on a suitably catalyzed substrate surface. At that point, which would otherwise be the termination point of the process for these non-autocatalytic solutions, an electrical current of negative potential is applied to the work-piece with the result that additional deposition of copper occurs. The electrical current can be applied immediately upon immersion of the work-piece in the electroless solution or at any time thereafter, but, for non-conductive substrates, no current flow or additional depositing will be realized until sufficient copper is electrolessly deposited from the solution onto the substrate by chemical reduction.
The method of U.S. Pat. No. 4,459,184 is quite useful in certain applications for building up thick copper deposits while still retaining advantages inherent in non-autocatalytic systems. However, its utility lies largely in the area where work-pieces can be individually racked during the depositing process. Where multiple racking is employed, such as the parallel multiple racking commonly used when coating printed circuit boards, the portion of the process of U.S. Pat. No. 4,459,184 which is electrolytic can result in uneven current distribution, causing burning in areas of high current density and insufficient coverage in areas of low current density.
Another approach to obtaining thick deposits from non-formaldehyde electroless copper solutions employing hypophosphite reducing agents is disclosed in U.S. Pat. No. 4,265,943 in which the inherently non-autocatalytic solutions of U.S. Pat. No. 4,209,331 are rendered autocatalytic by inclusion therein of a source of noncopper ions (e.g., nickel and/or cobalt ions) which function as autocatalysis promoters for the metallic copper deposition. Since the electroless solution is autocatalytic, utility for the common multiple racking arrangement of printed circuit boards is attained. A very similar process is set forth in U.S. Pat. No. 4,482,596 in which autocatalytic electroless coatings consisting of alloys of copper, cobalt or nickel, and phosphorus are disclosed. The solution from which this alloy coating is deposited is simply an electroless, formaldehyde-free copper depositing solution employing a hypophosphite reducing agent and containing a source of nickel or cobalt ions of the type disclosed in U.S. Pat. No. 4,265,943. The use of a number of so-called "autocatalytic promoters" also is disclosed. Apparently the phosphorus content of the alloy coating is solely attributable to the phosphorus inherently incorporated during deposition as a consequence of the use of the standard hypophosphite reducing agent in the solution, since no other source of phosphorus (other than certain disfavored promoters) is disclosed.
The starting point for the present invention began with work involving the autocatalytic, formaldehyde-free electroless copper solutions of U.S. Pat. No. 4,265,943, i.e., those in which a non-copper ion (e.g., nickel or cobalt ion) is employed as the promoter which causes the otherwise non-autocatalytic solutions to be autocatalytic. As noted, the solutions of U.S. Pat. No. 4,265,943 are quite useful for the manufacture of printed circuit boards wherein a thin copper deposit is electrolessly applied to a multiple racking of suitably catalyzed substrates, followed by application of a mask or resist over the coated substrate to define a desired printed circuit pattern. The masked substrate is then further plated in an electrolytic bath to build up additional metal thickness in unmasked regions. The mask is then dissolved and the board placed in a suitable copper etchant solution to remove the thin copper layer previously covered by the mask while retaining the thicker regions built up during electroplating. For such processes, as demonstrated by the examples of U.S. Pat. No. 4,265,943 wherein various non-conductive ABS plastic and fiberglass-reinforced epoxy resin substrates are copper coated, the patented process and solutions are quite useful and workable.
U.S. Pat. No. 4,265,943 also discusses applicability of the process and solutions therein to "subtractive" procedures for making printed circuit boards. In these procedures, copper foil clad epoxy laminates are provided with punched or drilled "through-holes" for interconnecting conductor areas on opposite surfaces of the laminate. Since the walls of the through-holes are required to be coated with copper for conductivity, the process and solutions of the patent, notwithstanding acknowledged passivity to pure copper, theoretically were useful for electrolessly depositing copper on the walls of the through-holes, with any further build-up of copper on the laminate surface per se being accomplished in a subsequent electrolytic plating step.
Notwithstanding the projected utility of the process and solutions of U.S. Pat. No. 4,265,943 for coating through-holes with copper, such utility was not borne out in practice. Thus, when a copper foil clad epoxy laminate for printed circuit applications, having throughholes, and suitably catalyzed for electroless deposition, was immersed in a solution according to the patent (i.e., source of copper ion, hypophosphite reducing agent, source of nickel or cobalt, ion, complexing agent and pH adjustor), not only did no deposition occur on the copper clad surfaces, but no deposition occurred on the walls of the through-holes either.
Another most interesting finding made while working with the autocatalytic solutions of U.S. Pat. No. 4,265,943 was that, even as the wholly non-conductive surfaces which can be easily continuously coated with thick deposits of copper according to the patent, an interruption of the depositing operation, such as caused by the presence of a contaminant or a significant drop in operating temperature or an intentional or unintentional removal of the work-piece from the solution, results in a termination of deposition which cannot be re-commenced simply by correction or elimination of the interrupting factor and re-establishment of the originally operable processing conditions.